This invention relates to an electronic color printing system and, more particularly, to a color printing system which utilizes a linear optical image bar to form two-color images on a photoreceptor surface in a single pass.
In a business office environment, it is frequently desirable to produce output copies at high speeds of original documents containing highlighted portions. Typically the output copies will have information in red and black, but could also be two different colors. It is well known in the light-lens document scanning art how to produce two color output copies using a multi-pass system, one commercial example being the Canon 3625 copier. For this type of system, an operator utilizes a mimic-type electronic edit pad to delineate areas of an original it is desired to highlight. The coordinates of a selected area are entered into machine memory. During a first exposure cycle, all areas on the photoreceptor are erased save the selected highlight color area which is then subsequently developed with the appropriate color toner. The resulting image is transferred to a copy sheet, fused and returned to the developer station entrance zone where it is re-registered. A second exposure of the original is made and the highlight area only is erased. The resulting latent image is developed with conventional black toner, and transferred to the copy sheet which is then fused and the copy sheet conveyed to an output tray.
Other types of light-lens scanning systems are known in the art for producing highlight color copies in a single pass mode. One approach, disclosed in U.S. Pat. Nos. 4,078,929 and 4,068,938, is to form three or more discrete levels of photoreceptor discharge. One level (high) corresponds to black information, an intermediate level corresponds to white background and the third level corresponds to a highlight color, such as red. Toner particles of appropriate polarity are then sequentially applied to the exposed image areas and transferred to a copy sheet to form a highlight colored print. Another technique exemplified by U.S. Pat. No. 4,562,129 is to use a bipolar photoreceptor comprising multiple layers, each layer sensitive to a different color. The two discharge areas are then developed by developer units biased to appropriate charge levels.
The above systems describe a single-pass highlight color system, which is used in a copying environment wherein an original document is scanned by a light source and the reflected image is projected onto the photoreceptor. It is desirable to produce a two-color output copy in a printing environment when an original document has been captured electronically and is to be reproduced by modulating light on a photoreceptor by a raster output scanning (ROS) device. Various techniques are known in electronic printing art to produce two-color output prints, as described below.
U.S. Pat. No. 4,403,848, assigned to same assignee as the present invention, discloses a printing system utilizing a flying spot scanner. This system uses a modulated laser ROS to form separate exposures at different locations of a photoreceptor belt, each exposure representing a specific color. The separate exposures are obtained by splitting the laser output into two beams by, for example, a wedge mirror. Each latent image is exposed with a characteristic color toner and transferred sequentially to a copy sheet. This system, shown in FIGS. 1 and 2 of the patent, can be characterized as a single pass, multiple image system. FIG. 3 of the same patent shows another color printing embodiment wherein three lasers are modulated in accordance with scanned document information and three imaging beams are scanned across the moving charged photoreceptor creating three color separation images which are developed in succession to form a composite color image. This color image is thereafter transferred to a copy sheet and fused to provide a color copy. For each of the two embodiments disclosed in this patent, the color images are formed by directing the modulated laser output to the surfaces of a rotating polygon which then reflects (scans) the image across the photoreceptor surface. The major problem with this type of prior art color printing system is the difficulty in registering the color images in both the single pass and multiple pass modes. The rotating mass of the polygon presents the main difficulty in maintaining precise registration since it is subject to mechanical speed irregularities and vibration.
Other single-pass ROS systems, which use the technique of splitting a laser output into two or more beams, are disclosed in U.S. Pat. Nos. 4,578,331 and 4,416,533 (FIG. 7). In both of these scanning systems, the separated beams are directed to separate areas of the photoreceptor with each image being developed by toner of the desired polarity and color. U.S. Pat. No. 4,791,452 is similar in operation, but uses two separate laser sources to produce the separate image exposures.
Still further examples of single-pass ROS printing systems are as follows: U.S. Pat. No. 4,731,634 discloses a single-pass ROS printing system which utilizes a photoreceptor having a plurality of discharge levels. A single exposure is provided to discharge the charge retentive surface of the photoreceptor to at least four levels. The exposed surfaces then move past the plurality of development stations where toner is attracted to the predetermined potential level. Also, see U.S. Pat. No. 4,509,850 which discloses a single-pass printer wherein a laser output is directed to a photoreceptor surface to record either a modulated charged area or a continuously charged area. Two developer rolls apply toner of appropriate polarity to their respective charged surfaces.
An attractive alternative scanning system to the polygon ROS type of system disclosed in the above patents is the use of linear optical image bars. Optical image bars, as known in the art, comprise an array of optical picture elements (pixels), for converting a bit map pattern, usually represented by the information contents of electrical input data signals, into a corresponding spatially varying profile. Although there are a variety of applications for these image bars in a number of different fields, a significant portion of the effort and expense that have been directed to their development has been directed towards their application to electrophotographic printing where they provide a relatively low cost and reliable alternative to the flying spot raster scanners of the type disclosed in the above discussed patents. The optical image bar generates line-like image ray patterns, representative of the image signal input, on the surface of a photoreceptor. One example of an image bar type of printer is found in U.S. Pat. No. 4,477,175, where the image bar is composed of a plurality of individually activated illumination elements (LEDs). Another example is found in U.S. Pat. No. 4,367,925 where the image bar is composed of a plurality of individually activated electro-optic electrodes. Also known as an EO TIR (electro-optic total internal reflection) spatial light monitor. This type of device characteristically comprises a plurality of laterally separated, individually addressable electrodes which are maintained on, or closely adjacent to, a reflective surface of an optically transparent electro-optic element, such as a lithium niobate (LiNbO.sub.3) crystal. In operation, substantially the full width of the electro-optic element of such a modulator is illuminated by a linearly polarized, transversely collimated light beam. Thus, when voltages representing the pixels of a linear pixel pattern (e.g., the pixels for a given line of an image) are applied to its individually addressable electrodes, the modulator spatially phase modulates the wavefront of the light beam in accordance with the applied pixel pattern. As a general rule, of course, the spatial wavefront modulation varies as a function of time in accordance with the pixel patterns for successive lines of a two dimensional image, thereby providing a line-by-line representation of the image.
The same advantages in speed are desirable in images by ROS systems. Applicants are aware of only one prior art embodiment of a single-pass image bar highlight color system disclosed in copending application U.S. Ser. No. 07/432,012 filed on Nov. 6, 1989 and assigned to the same assignee as the present invention. In that application, two image bar outputs are directed either simultaneously onto two photoreceptors with two associated development systems, or sequentially onto the same photoreceptor via belt transfer development system. It would be desirable to enable a single-pass color highlight system without requiring the use of two image bar ROS systems or the lower printing speed associated with the sequential embodiment.
According to the present invention, it has been found that the on-time of an image bar is separated into two equal time intervals; each time interval is used to write one image requiring one color development on the photoreceptor, while the other half of the line time is used to write another image of a second color, requiring a second color development in the other half. The separated exposed images are sequentially developed by toner of appropriate color. In one embodiment, an image bar of the type disclosed in U.S. Pat. No. 4,367,925 is adapted to transmit either S polarized light or P polarized light during one half of the raster on-time. In a second embodiment, an LED image bar output is split and directed through one or the other of a pair of linear gradient index lens arrays to separate areas of the photoreceptor. More particularly, the invention relates to a single-pass highlight color printer for forming line images on the surface of a photoreceptor including, in combination, a single image bar ROS for exposing two separate areas of a photoreceptor at two exposure stations, each exposure occurring during one-half of the on-time of said ROS, means for applying a charge potential to said exposure areas prior to said exposure, means for developing each exposed area with a developer of a different color, and means for transferring said developed image onto a copy substrate to form a two-color output copy .